This invention relates to a method for producing a data carrier having at least in partial areas a transparent plastic layer which is provided at least in a partial area with a surface relief in the form of a lens structure. Further the invention relates to a data carrier produced by this method and to an apparatus for carrying out the method.
There have been efforts for some time to provide data carriers such as ID cards, credit cards or papers of value with special security features having a striking, uncopiable visual effect, on the one hand, and ensuring cost-effective production, on the other hand.
For example, EP 0 219 012 A1 discloses a multilayer data carrier with a transparent cover foil in which a surface relief in the form of a lens structure, preferably a cylinder lens screen, is incorporated. Through this lens screen information is incorporated by means of a laser in subjacent volume areas of the data carrier, being visually well recognizable as blackened areas. By reason of the focal effect of the lenses, only narrowly limited areas of the data carrier are blackened so that the information can only be observed at the viewing angle corresponding to the angle of incidence of the laser on the lens structure. In this way one can inscribe several pieces of information recognizable only at certain viewing angles by using different inscription angles. This effect will be called a xe2x80x9ctilt imagexe2x80x9d in the following.
This tilt image has a variety of security-related advantages. For example it is reproducible neither with photographic means nor with copying technology, since the tilt image information is never all present at one camera angle simultaneously. The false data carrier consequently only shows one part of the information and this at all viewing angles. The tilt image effect has vanished. Moreover the laser inscription offers additional protection from forgery. Laser inscription produces visible changes in the interior of the data carrier material that cannot be removed or altered either chemically or mechanically without completely destroying the data carrier.
This security feature also offers some advantages with respect to economical production of data carriers. Inscription of the lens screen area takes place only after completion of the data carrier, so that any rejects that occur are not inscribed and complicated runs for subsequently producing a new data carrier with the same data record are therefore avoided. This holds in particular if the tilt image information is user-related.
The lens structure was hitherto produced by embossing with an embossing die either during production of the data carrier or after its completion. However the heating of the data carrier material and the pressure exerted thereupon can easily cause irreparable distortions and disturbing embossings through to the back of the data carrier, which is frequently used as a printing surface.
Therefore the lens structures are preferably incorporated during lamination of the data carriers by providing one of the laminating plates with the negative lens screen structure. Since lamination is usually done in sheet format, the laminating plate must have the suitable negative relief structure in the area of each individual copy. This has the disadvantage that the entire laminating plate must be replaced as soon as an error occurs in the area of one copy.
The invention is therefore based on the problem of proposing a method for providing data carriers with surface structures, in particular lens structures, without distortions and economically even in short runs without the stated disadvantages of the prior art.
Solutions to this problem are stated in the independent claims. Developments can be found in the respective sub-claims.
According to a preferred embodiment the relief structure is incorporated with the aid of accordingly formed cutting tools, such as planing or grinding tools, in the data carrier consisting of a transparent plastic in the area of the lens structure to be incorporated. As apparent from polished samples, this removal method can produce a very smooth lens surface with good optical quality. This method is suitable for all conventional data carrier materials, in particular thermostable materials such as PC with which usual embossing methods cannot be applied. By reason of the material removal, the data carrier produced by this method has a lens structure slightly sunk in from the data carrier surface, i.e. the thickness of the data carrier is not altered by the lens structure as with known data carriers. This fact is of great advantage in particular for stacking the inventive data carriers. By sinking or quasi embedding the lens structure in the data carrier one moreover protects the lens surface optimally against wear due to environmental influences.
According to a further embodiment one can also use individual matrixes working on a given stepwise heating and pressure cycle in order to avoid distortions of the data carrier material. For this method one preferably uses an apparatus having several embossing units working in parallel. When an individual data carrier runs into the apparatus an embossing die is mounted on the data carrier, being jointly moved with the card along a given, for example circular, transport path. During transport through the embossing apparatus the jointly moved embossing die is heated in accordance with the given working cycle and cooled after the data carrier material has slowly softened and been put in the desired form. Jointly moving the heating and cooling device has the advantage that the entire transport time can be utilized as process time. Alternatively the heating/cooling device can also be stationary, however, which has the advantage that the temperature can be adjusted in central and stationary fashion for the entire apparatus. The data carriers are thereby clocked through the stationary heating/cooling units together with the mounted embossing dies.
The option of being able to control the temperature and pressure cycle within wide limits without having to accept any great disadvantages with respect to productivity, also makes it possible to produce by this method data carriers wherein the lens structure leads to no increase in the data carrier thickness. Due to the slow change of state the data carrier material can be deformed very uniformly and without distortions and the surplus data carrier material distributed uniformly over the entire data carrier.
In order to minimize the risk of distortions further, one can provide the data carrier surface before the embossing process with a layer of lacquer which is thermoplastically deformable during embossing and only completely cured afterwards. The cured lacquer layer acts as a kind of fixing layer for the subjacent thermally softened zone of the data carrier material, which can cool off completely without distortion after the actual embossing process and cure in the form given by the lacquer layer. However it is also possible to emboss only into the lacquer layer. In this case the danger of distortions is averted completely since the data carrier material itself is not stressed during embossing.
If the data carrier is to be equipped with a locally limited tilt image element, distortions and signs of wear can also be avoided by producing the tilt image element as a separate single element and then incorporating it into an accordingly dimensioned gap in the data carrier.
The lens element can be produced for example by a continuous embossing method wherein a transparent foil is provided with the desired structure by web-fed embossing. It is likewise conceivable to equip the foil with the suitable surface contour already during extrusion. The foil is then punched into the single elements and fastened in the data carrier gap by gluing or welding for example. A further possibility is offered by injection molding, with which single elements of the desired form can be produced directly.
Embodiments are also possible in which the data carrier or parts thereof are produced by injection molding. For example one can injection mold a data carrier having the lens structure locally limited on one of its surfaces, the apex areas of the lens structure preferably ending flush with the data carrier surface. On the opposite side of the data carrier one provides below the lens structure a gap in which a separately produced plastic element is inserted. This element is specially adapted to the requirements of laser inscription since either the element material is mixed with additives which absorb laser radiation, or the element is coated with an accordingly prepared lacquer layer on the side facing the data carrier.
A further variant is to injection mold a plastic foil at least on one side with a translucent layer having the lens structure in a partial area. If the foil is provided by injection molding with a cover layer on both sides, the second cover layer can also have a lens structure in a certain area. In a special embodiment the lens structures can be disposed congruently or at least concentrically with each other. If necessary the plastic foil can be provided, before the cover foils are applied, with a coating sensitized for absorbing laser radiation in the area of the lens screen to be applied.
The latter methods can do completely without complicated and elaborate embossing apparatuses within the production line of the data carriers. This permits more economical and efficient production. Producing continuously embossed foils or injection molding single elements is cost-effective since one can do long runs in a continuous process that need only be adapted to the boundary conditions for this one process. There are so longer problems such as embossing through to the back of the data carrier or distortions.
The inventive methods further have the advantage that they can be performed on the card when its layer structure is finished, so that problems with rejects are easy to handle. If a processing step has been improperly executed the data carrier in question can be immediately eliminated and replaced with a following one.
Further advantages and advantageous embodiments will be explained with reference to the figures. It is pointed out that the figures only sketch the essential method steps and are not to be understood either as complete or as true-to-scale representations of the individual apparatuses.